Traditionally, as a TFT (Thin Film Transistor) of an active matrix substrate in a display device such as a liquid crystal display device, a non-crystalline (amorphous) silicon TFT, a microcrystalline silicon TFT, a polycrystalline silicon (polysilicon) TFT, and the like have been used.
The amorphous silicon TFT is suitable for a TFT of a display device that requires a large surface because it is possible to form an amorphous silicon film in a relatively simple manner. Therefore, many active matrix substrates that are used in liquid crystal televisions having relatively large screens use the amorphous silicon TFTs.
The microcrystalline silicon TFT and the polycrystalline silicon TFT have high mobility of electrons and holes in semiconductor layers, and have high ON currents. Because of this, they have an advantage of being able to charge a pixel capacitance of a liquid crystal display device or the like in short switching time. Further, when the microcrystalline silicon TFT and the polycrystalline silicon TFT are used, there is also an advantage that a peripheral circuit such as a driver can be built in an active matrix substrate partially or completely.
An example of a semiconductor device that includes a polysilicon TFT is described in Patent Document 1. FIG. 10 shows a cross-sectional configuration of the semiconductor device according to Patent Document 1.
As shown in FIG. 10, the semiconductor device of Patent Document 1 has a polysilicon TFT 1000A and an amorphous silicon TFT 1000B that are formed on a single substrate 1001. The polysilicon TFT 1000A and the amorphous silicon TFT 1000B are both bottom gate type TFTs having a reverse stagger structure, and are formed on the substrate 1001 at the same time. The polysilicon TFT 1000A is used as a TFT for a driver circuit in a liquid crystal display device. The amorphous silicon TFT 1000B is used as a switching element inside a pixel of the liquid crystal display device.
Both the polysilicon TFT 1000A and the amorphous silicon TFT 1000B are constituted of a gate electrode 1002, a gate AO film (anode oxidation film) 1003, a gate insulating film 1004, an active layer, an etch stopper 1020, a source region 1030 and a drain region 1031 that are semiconductor layers containing an impurity, a source electrode 1040, and a drain electrode 1041, all of which are formed on the substrate 1001. The active layer of the polysilicon TFT 1000A is a polysilicon active layer 1010, and the active layer of the amorphous silicon TFT 1000B is an amorphous silicon active layer 1011.
When manufacturing this semiconductor device, a portion of amorphous silicon formed on the gate insulating film 1004 becomes the amorphous silicon active layer 1011. Another portion is irradiated with an excimer laser or light of a halogen lamp to be crystallized, thereby turning into the polysilicon active layer 1010. In the polysilicon TFT 1000A, the source region 1030 and the drain region 1031 are laminated after the polysilicon active layer 1010 has been formed from the amorphous silicon.